Security forces have killed nine militants in the Sinai Peninsula and also destroyed 15 vehicles transporting weapons into Egypt across its western border, the military said in statements on Sunday.

The "extremists" were killed in two separate incidents in the middle Sinai, but the military did not say when they took place.

Both involved the air force and "law enforcement forces".

The military said a planned "hostile act" was foiled after "a highly dangerous terrorist" group was discovered and tracked, and six militants were killed in a mountainous area.

In a separate incident, three "highly dangerous" extremists were killed and another was arrested.

A four-wheel-drive vehicle and five caches containing "large amounts of explosives and ammunition" were destroyed, the military added.

On Egypt's western border with Libya, warplanes have attacked and destroyed 15 four-wheel-drive vehicles carrying weapons, ammunition and contraband over the past 24 hours, the military said on Sunday.

The air strikes took place following intelligence that "criminal subjects were gathering to sneak into Egypt", it said.

The military has boosted its operations on the border, repeatedly expressing concern about militants crossing the frontier to launch attacks.

Libya has been wracked by chaos since the 2011 uprising that toppled and killed longtime dictator Moamer Kadhafi, with rival authorities and militias battling for control of the oil-rich country.

In May, Egyptian President Abdel Fattah al-Sisi said setbacks by the Islamic State group (IS) in Syria were driving its fighters to try to relocate to Libya and the Sinai.

The same month, the air force attacked "terror camps" in Libya in retaliation for a deadly attack on Coptic Christians in Egypt, saying the assailants had been trained there.

Egypt's military is battling a local affiliate of IS which has waged a deadly insurgency in the northern Sinai that has killed hundreds of members of the security forces.

Since December, IS has also targeted Christians in other parts of the country, including in Cairo, killing dozens.

The jihadists have threatened to carry out further attacks on Christians, who make up about 10 percent of Egypt's 90 million people.